1. Field of the Invention
The present invention relates to a control system and method for parallel hybrid vehicles in which cylinders can be deactivated, and in particular, a control system for improving driving performance (called xe2x80x9cdriveabilityxe2x80x9d) when the vehicle is reaccelerated from a cylinder-deactivated engine operation while considering the driver""s intention.
2. Description of the Related Art
Conventionally, hybrid vehicles employing an engine and a motor as driving sources are known. Among them, in parallel hybrid vehicles, the output of the engine is assisted by driving the motor.
More specifically, when such a parallel hybrid vehicle is accelerated, the engine output is assisted by driving the motor, and when decelerated, various controls such as battery charging using deceleration regeneration are performed so as to maintain a necessary remaining battery charge (i.e., necessary electrical energy) and also to satisfy the driver""s intention. In addition, the parallel hybrid vehicle has a structure in which the engine and the motor are serially arranged; thus, the structure can be simplified so that the total weight of the system can be reduced and the engine and the motor can be flexibly arranged in the vehicle.
In order to remove effects due to the friction of the engine (i.e., engine braking) during deceleration regeneration, a parallel hybrid vehicle having a clutch between the engine and the motor (refer to Japanese Unexamined Patent Application, First Publication No. 2000-97068) or a parallel hybrid vehicle having a structure in which the engine, motor, and transmission are directly and serially coupled are known.
However, in the former example having clutch between the engine and the motor, the structure employing the clutch is complicated and thus the flexibility in arranging each of the structural elements is reduced. Also in this structure using the clutch, the transmission efficiency of the power transmission system is reduced even when the vehicle is running.
On the other hand, in the latter example having a structure in which the engine, motor, and transmission are directly and serially coupled, the amount of regeneration is reduced due to the above-described engine friction; thus, the energy which can be obtained by the regeneration is reduced, so that the amount of driving power assistance by the motor is limited.
Additionally, in the former example, in order to reduce the engine friction during deceleration, an electronic control throttle mechanism may be employed for opening the throttle valve during deceleration so as to considerably reduce pumping loss, thereby increasing the amount of regeneration. However, in this method, a large amount of pre-used air is directly drawn into the exhaust system during deceleration, so that the temperatures of the catalyst and A/F sensor (air-fuel sensor) are decreased and exhaust gas control may not be suitably performed.
In order to solve this problem, a cylinder deactivation technique has become the focus of attention; however, in this case, it is difficult to smoothly switch the driving operation from a cylinder-deactivated engine operation to an all-cylinder-activated engine operation while considering the driver""s intention.
In consideration of the above circumstances, an object of the present invention is to provide an engine control system and method for hybrid vehicles, which uses a cylinder deactivation technique and can smoothly switch the driving operation from the cylinder-deactivated engine operation to the all-cylinder-activated engine operation while considering the driver""s intention.
Therefore, the present invention provides an engine control system in a hybrid vehicle including an engine (e.g., an engine E in an embodiment explained below) and a motor (e.g., a motor M in the embodiment explained below) as driving sources, wherein when the vehicle is decelerated, a cylinder-deactivated engine operation, in which at least a part of the total number of cylinders of the engine is deactivated, is performed and regeneration using the motor is performed, the system comprising:
a cylinder deactivation release determining section (e.g., an FIECU 11 in the embodiment explained below) for releasing the cylinder-deactivated engine operation when an engine speed decreases to a reference engine speed (e.g., a lower-limit engine speed NDCSL (for continuing the cylinder-deactivated engine operation) in the embodiment explained below) with respect to the cylinder-deactivated engine operation; and
a brake operation detecting section (e.g., a brake switch S4 in the embodiment explained below) for detecting whether a brake pedal is depressed, wherein
the cylinder deactivation release determining section includes a cylinder deactivation condition changing section for changing the reference engine speed according to whether the brake pedal is depressed, which is detected by the brake operation detecting section.
According to the above structure, it is possible to detect whether the brake pedal is depressed, by the brake operation detecting section, and to estimate whether the driver intends to stop the vehicle, and based on the estimation, the reference engine speed can be suitably determined by the cylinder deactivation condition changing section. For example, when the brake pedal is depressed, the driver probably intends to stop the vehicle; therefore, the reference engine speed may be set to a lower value. Conversely, when the brake pedal is not depressed, the vehicle may not be stopped and may be reaccelerated; thus, the reference engine speed may be set to a higher value so as to ensure the driveability necessary for reacceleration. Accordingly, the cylinder-deactivated engine operation can be smoothly switched to the all-cylinder-activated engine operation while considering the driver""s intention.
As a typical example, the cylinder deactivation condition changing section changes the reference engine speed according to a temperature of an engine oil (e.g., xe2x80x9cTOILxe2x80x9d in the embodiment explained below). Accordingly, the reference engine speed can be changed according to the temperature of the engine oil, that is, according to how easily the pressure is applied, which depends to some extent on the viscosity of the oil. Therefore, highly accurate control can be performed according to the thermal conditions of the engine.
As another typical example, the cylinder deactivation release determining section releases the cylinder-deactivated engine operation when a vehicle speed decreases to a reference lower-limit vehicle speed (e.g., a lower-limit brake-off vehicle speed #VPDCSL or a lower-limit brake-on vehicle speed #VPDCSBL in the embodiment explained below) with respect to the cylinder-deactivated engine operation; and
the cylinder deactivation condition changing section changes the reference lower-limit vehicle speed according to whether the brake pedal is depressed, which is detected by the brake operation detecting section.
Accordingly, it is possible to detect whether the brake pedal is depressed, by the brake operation detecting section, and to estimate whether the driver intends to stop the vehicle, and based on the estimation, the reference lower-limit vehicle speed can be suitably determined by the cylinder deactivation condition changing section. For example, when the brake pedal is depressed, the driver probably intends to stop the vehicle; therefore, the reference lower-limit vehicle speed may be set to a lower value. Conversely, when the brake pedal is not depressed, the vehicle may not be stopped and may be reaccelerated; thus, the reference lower-limit vehicle speed may be set to a higher value so as to ensure the driveability necessary for reacceleration. Accordingly, the cylinder-deactivated engine operation can be smoothly switched to the all-cylinder-activated engine operation while considering the driver""s intention.
The present invention also provides an engine control method used in a hybrid vehicle including an engine and a motor as driving sources, wherein when the vehicle is decelerated, a cylinder-deactivated engine operation, in which at least a part of the total number of cylinders of the engine is deactivated, is performed and regeneration using the motor is performed, the method comprising:
a cylinder deactivation release determining step of releasing the cylinder-deactivated engine operation when an engine speed decreases to a reference engine speed with respect to the cylinder-deactivated engine operation; and
a brake operation detecting step of detecting whether a brake pedal is depressed, wherein
the cylinder deactivation release determining step includes a cylinder deactivation condition changing step for changing the reference engine speed according to whether the brake pedal is depressed, which is detected in the brake operation detecting step.
The cylinder deactivation condition changing step may include changing the reference engine speed according to a temperature of an engine oil.
As an typical example, the cylinder deactivation release determining step includes releasing the cylinder-deactivated engine operation when a vehicle speed decreases to a reference lower-limit vehicle speed with respect to the cylinder-deactivated engine operation; and
the cylinder deactivation condition changing step includes changing the reference lower-limit vehicle speed according to whether the brake pedal is depressed, which is detected in the brake operation detecting step.